1. Field of the Invention
The present invention relates to an optical beam control device, in particular, to an optical beam control device that controls an optical beam used to expose an image bearing member such as a photosensitive member in an image-forming device, and to an image-forming device including the same.
2. Description of Prior Art
In an electrophotographic type of image-forming device, an optical beam is emitted by an optical beam emission device such as a laser diode, and the thus-emitted optical beam is deflected by optical components such as a polygon mirror. The optical beam then forms a latent electrostatic image by scanning the surface of a photosensitive member that has been uniformly charged. Therefore, an image is formed on a recording sheet through processing such as development and transfer.
The method for controlling the luminous energy of the optical beam is an auto power control (APC) method that involves using a luminous energy detection sensor to detect the luminous energy of the emitted light, such as a PIN-type photodiode, and then controlling the drive of the laser diode in response to the detected luminous energy. However, a problem arises in that, if a large amount of current starts flowing into the laser diode immediately after the activation of the laser diode, variations in the characteristics of the laser may result in exceeding the rated value, which may lead to destruction of the laser diode, as described in Japanese unexamined patent publication 2-96786.
In addition, the control over the optical beam relates to the timing of the light emission as well as to the luminous energy as described above. Control of the light-emission timing involves the use of an optical beam detection sensor (BD sensor) that is provided at the position that the optical beam passes at a timing just before a scan of the surface of the photosensitive member. In other words, the control is performed in order that the timing at which the optical beam passes the BD sensor is detected, and the optical beam then exposes a proper position on the surface of the photosensitive member, based on an output signal of the BD sensor (BD detection signal).
Japanese unexamined patent publication 2-96786 discloses a luminous energy control device that controls the luminous energy so as to increase gradually from the activation of the laser diode, thereby preventing destruction of the components such as the laser diode. In addition, the laser diode is controlled to emit light only at a timing at which the surface of the photosensitive member is not directly exposed, after the optical beam has reached a predetermined amount of luminous energy at which the BD sensor can detect the optical beam.
However, the above-described control device has problems concerning deterioration of the laser diode and deterioration of the surface of the photosensitive member scanned by the optical beam, since the laser diode always emits a light beam until the BD sensor detects the optical beam.
In order to solve the above problems, an main object of the present invention is to provides an optical beam control device that can suppress deterioration of the laser diode and photosensitive member, and an image-forming device that uses the optical beam control device.